Charged particle beam device, sample position information calculation method therefor, sample shape estimation method, and sample position adjustment method

Abstract

A charged particle beam device comprises: a sample stage (21) for holding a sample (20) that includes an evaluation region; charged particle optics for irradiating the sample with a charged particle beam; a detector (31) for detecting signal electrons emitted in response to the interaction between the sample and the charged particle beam; and a control system (42) for controlling the sample stage, the charged particle optics, and the detector. The control system calculates a position displacement amount of the evaluation region from a reference height on the basis of the magnification of an observation image, of the evaluation region, generated on the basis of the signal electrons detected by the detector.

Description

Charged Particle Beam Device, Method for Calculating Sample Position Information, Method for Estimating Sample Shape, and Method for Adjusting Sample Position

[0001] The present invention relates to a charged particle beam device, a method for calculating sample position information thereof, a method for estimating sample shape, and a method for adjusting sample position.

[0002] Patent Document 1 discloses magnification display correction for a transmission electron microscope (TEM). According to Patent Document 1, in a transmission electron microscope, since the sample position is slightly deviated from the reference position due to sample desorption, the display magnification in terms of the film surface of the electron microscope includes an error based on the deviation from the reference position in the optical axis direction of the preset sample. Therefore, the amount of deviation of the sample from the reference position in the optical axis direction is obtained from the change amount of the objective lens current accompanying focusing, the magnification error of the objective lens is calculated from the amount of deviation from the reference position, and the display magnification displayed on the display is corrected based on the magnification error.

[0003] Japanese Patent Laid-Open No. 11-135053

[0004] In order to perform high-precision length measurement of a sample based on a TEM image taken by a transmission electron microscope, high absolute accuracy of magnification and high reproducibility are required.

[0005] According to the technique disclosed in Patent Document 1, the magnification caused by the deviation of the sample from the reference position in the optical axis direction is corrected by using the change amount of the objective lens current accompanying focusing. However, since the magnetization state of the lens magnetic path of the objective lens has hysteresis, even when the same objective lens current is applied, the actual magnification varies depending on the magnetic history such as the magnification adjustment history for the objective lens. Therefore, the correction accuracy by the method of Patent Document 1 includes an error due to the hysteresis of the objective lens, and is insufficient particularly when high length measurement accuracy is required.

[0006] A charged particle beam apparatus according to one embodiment of the present invention comprises a sample stage for holding a sample including an evaluation region, a charged particle optical system for irradiating the sample with a charged particle beam, a detector for detecting signal electrons emitted as a result of interaction between the sample and the charged particle beam, and a control system for controlling the sample stage, the charged particle optical system, and the detector. The control system calculates the amount of positional displacement of the evaluation region from a reference height based on the magnification of the observation image of the evaluation region generated based on the signal electrons detected by the detector.

[0007] This invention provides a charged particle beam apparatus that achieves high magnification, absolute accuracy, and high repeatability. Other challenges and novel features will become apparent from the description herein and the accompanying drawings.

[0008] This is a schematic diagram showing the basic configuration of a transmission electron microscope. This is an example of the computer hardware configuration. This is a flowchart of the TEM image acquisition process. This diagram explains the structure included in the evaluation region and the method for acquiring scale data. This diagram explains the structure included in the evaluation region and the method for acquiring scale data. This diagram explains the structure included in the evaluation region and the method for acquiring scale data. This diagram explains the structure included in the evaluation region and the method for acquiring scale data. This is a graph showing the relationship between the amount of sample displacement in the optical axis direction and the magnitude of the magnification fluctuation of the observed image, as shown by the correlation data. This diagram explains the reference height. This is a plan view and a side view of the sample held on the sample stage. This is a flowchart for calculating the amount of ROI displacement in the optical axis direction considering the inclination of the sample. This is a flowchart for acquiring correlation data. This is an example of correlation data.

[0009] Figure 1 schematically shows the basic configuration of a transmission electron microscope. The TEM includes an imaging unit 51 and a control system 42. The imaging unit 51 comprises an electron optical system and a camera 31 as a detector, with an electron source 1, an extraction electrode 2, a condenser lens 11, a condenser diaphragm 12, an objective lens 22, a limiting field diaphragm 23, and an imaging lens 30 as its main components. The sample 20 is a thin film sample held on a sample stage 21 and inserted between the upper and lower magnetic poles of the objective lens 22.

[0010] The primary electron beam (probe) generated by the electron source 1 is extracted by the extraction electrode 2 and irradiated toward the sample 20. The condenser lens 11 adjusts the focusing conditions of the primary electron beam. The objective lens 22 adjusts the focus position of the primary electron beam relative to the sample 20. The limiting field diaphragm 23 is an aperture that limits the area of ​​the sample when displaying the electron diffraction pattern, and is inserted into the image plane of the objective lens 22. The imaging lens 30 uses the transmitted electrons emitted from the surface of the sample 20 opposite to the surface on which the primary electron beam was incident as signal electrons, and projects the image formed at the position of the limiting field diaphragm 23 onto the detection surface of the camera 31. The camera 31 captures the image of the transmitted electron beam (TEM image) formed on the detection surface.

[0011] The control system 42 includes a control device 40 and a computer 41. The computer 41 stores control programs and image processing programs for the imaging unit 51, and by executing these programs, the computer 41 functions as a control unit that controls the imaging unit 51, or as an image processing unit that processes the TEM images generated by the imaging unit 51. The control device 40 receives control signals from the computer 41 and controls each component of the imaging unit 51. The control device 40 includes an AD converter, a DA converter, memory, and an arithmetic unit such as an FPGA or microprocessor.

[0012] Figure 2 shows an example of the hardware configuration of computer 41. Computer 41 includes a processor 43, memory (main memory) 44, auxiliary storage 45, output device 46, input device 47, and communication interface (I / F) 48. These components are connected to each other via a bus 49 so that they can communicate with one another.

[0013] Memory 44 is composed of, for example, semiconductor memory and is mainly used to hold the currently running program and data. The processor 43 performs various processes according to the program stored in memory 44. Various functional units are realized by the processor 43 operating according to the program. The auxiliary storage device 45 is composed of, for example, a large-capacity storage device such as a hard disk drive or a solid-state drive and is used to hold programs and data for a long period of time. The program and data stored in the auxiliary storage device 45 are loaded into memory 44, and the processor 43 executes the program, thereby executing various processes of the computer 41.

[0014] The input device 47 is a hardware device for the operator to input instructions and information into the computer 41. The output device 46 is a hardware device that displays various images for input and output, such as a display device or a printing device. The communication interface 48 is an interface for connecting to a network.

[0015] Prior to the imaging unit 51 taking a TEM image, the electron optical system is adjusted. The adjustment of the electron optical system includes adjusting the position of the sample 20 (sample position adjustment). The TEM image acquisition process will be explained using Figure 3. This process is configured to achieve high absolute accuracy and high reproducibility with high magnification, enabling highly accurate measurement of the sample length, and is controlled by the control system 42. First, the imaging lens 30 is excited to a predetermined excitation amount (S01). The magnification at which the image formed at the position of the limiting field aperture 23 is projected onto the detection surface of the camera 31 corresponds to the excitation amount of one or more imaging lenses 30 constituting the imaging system, so an excitation current corresponding to the set magnification is applied to the imaging lens 30. It is desirable to demagnetize the imaging lens 30 prior to adjusting the electron optical system so that the magnetic history of the imaging lens 30 does not affect the magnification.

[0016] A primary electron beam is irradiated from the electron source 1 onto the sample 20 held by the sample stage 21. The XY movement mechanism of the sample stage 21 is used to select a field of view for position adjustment, and the objective lens 22 is used to adjust the focus (S02), and an observation image is acquired (S03). The field of view for position adjustment is a field of view for acquiring an observation image to adjust the position of the sample 20 in the optical axis direction (Z direction), and a region (evaluation region) in which the magnification of the observation image can be accurately calculated is selected. Subsequently, scale data is acquired from the acquired observation image (S04). The scale data is, for example, the magnification of the observation image. By selecting a region containing a structure with known dimensions as the evaluation region, the magnification of the observation image can be calculated from the dimensions of the image of that structure in the observation image.

[0017] Figures 4A to 4D illustrate structures that can be used to acquire scale data. Figure 4A is a crystal lattice image. The evaluation region is defined as the region where the crystal lattice is visible, and the lattice fringe spacing d1 is used as a known dimension. Figure 4B is the Fourier transformed image of the crystal lattice image. The periodicity of the crystal lattice is reflected in the position of the diffraction spots 62 in the Fourier transformed image 61. The spacing between diffraction spots can be used to calculate the magnification of the observed image. Similarly, scale data may be acquired by obtaining an electron diffraction image of the crystal lattice. Structures with known dimensions are not limited to crystal lattices; scale data may also be acquired by using the length d3 of one side of any pattern 63 with known dimensions, as shown in Figure 4C, or the pitch d4 of a line-and-space pattern 64, as shown in Figure 4D.

[0018] Next, based on the scale data acquired in step S04, the amount of displacement of the sample 20 in the optical axis direction (Z direction) is calculated (S05). For this purpose, correlation data between the amount of displacement of the sample in the optical axis direction and the magnification fluctuation of the observed image is used. The correlation data is acquired in advance and stored in the auxiliary storage device 45. Figure 5 is a graph showing the relationship between the amount of displacement of the sample in the optical axis direction and the magnitude of the magnification fluctuation of the observed image, as indicated by the correlation data. The correlation data stored in the auxiliary storage device 45 may be in the form of a table, for example, or in the form of a mathematical formula.

[0019] In the graph of Figure 5, the horizontal axis represents the amount of displacement of the sample in the optical axis direction from the reference height, and the vertical axis represents the actual magnification ratio, which is the magnification of the observed image normalized to 1 when the magnification is at the reference height. The reference height can be set as the focal position when the objective lens 22 is excited with a predetermined excitation amount. The method for creating the graph of Figure 5 will be described later. As shown in Figure 6, the actual magnification ratio gradually increases from 1.00 as the sample 20 moves away from the reference height in the Z+ direction, and gradually decreases from 1.00 as the sample 20 moves away from the reference height in the Z- direction. This is because the size of the image formed at the position of the limiting field aperture 23 changes. Therefore, the actual magnification ratio of the observed image (= magnification of the observed image / set magnification) is calculated from the scale data (magnification) of the observed image. If |(actual magnification ratio - 1)| < α (No in S06), then no adjustment of the position of the sample 20 in the optical axis direction is required. If |(actual magnification ratio - 1)| ≥ α (Yes in S06), then the position of the sample 20 in the optical axis direction is adjusted (S07). Here, α is set as the allowable value for the amount of misalignment in the optical axis direction. Note that the relationship between the amount of misalignment in the optical axis direction of the sample and the magnification fluctuation of the observed image depends on the configuration of the optical system. As the sample 20 moves away from the reference height in the Z+ direction, the actual magnification ratio gradually decreases from 1.00, and as the sample 20 moves away from the reference height in the Z- direction, the actual magnification ratio may gradually increase from 1.00.

[0020] In step S07, the Z-movement mechanism of the sample stage 21 is controlled to compensate for the positional displacement calculated in step S05, so that the sample 20 is positioned at the reference height.

[0021] After controlling the sample stage 21 (step S07), the process from step S03 is executed again to confirm that the sample 20 is correctly positioned at the reference height. Once the adjustment of the electron optical system is complete, the field of view for acquiring the observation image is moved to the region of interest (ROI) using the XY movement mechanism of the sample stage 21 (S08). Note that this step is unnecessary if the ROI coincides with the evaluation area. Next, focus adjustment is performed using the objective lens 22 (S09). In the case of TEM images, for example, if the position of the sample in the optical axis direction is slightly off from the just-focus position, an image with stronger contrast may be obtained. In such cases, the sample 20 is shifted from the reference height using the Z movement mechanism of the sample stage 21 (S09) to obtain an image with the desired contrast. In this case, the amount of shift from the reference height is stored in the auxiliary storage device 45, and the same amount of shift is used to adjust the focus for samples being measured. The image magnification changes as the position of the sample 20 in the optical axis direction shifts from the reference position. Therefore, by using the same amount of shift, the amount of change in image magnification can be made equal, and a decrease in the reproducibility of magnification can be suppressed. Subsequently, the observation image of the ROI is acquired (S10).

[0022] If the evaluation region and the ROI are located at the same position in the optical axis direction in sample 20, the above adjustments are considered sufficient. However, if, for example, sample 20 is not positioned perpendicular to the optical axis of the electron optical system, and the evaluation region and the ROI are located at different positions on the XY plane, then it is necessary to adjust the amount of displacement in the optical axis direction in the ROI, taking into account the inclination of sample 20.

[0023] Figure 7 schematically shows a plan view and a side view of the sample 20 held on the sample stage 21. The plan view is a view of the sample 20 from the optical axis direction (Z direction), and the side view is a view of the sample 20 from a direction perpendicular to the optical axis direction (e.g., the Y direction). In this example, the ROI 71 is located in the center of the sample 20, while the evaluation regions 72 and 73 are located at the edges of the sample 20. In this case, even if the position of the ROI 71 in the optical axis direction is adjusted to match either the evaluation region 72 or the evaluation region 73, a relatively large deviation from the reference height remains.

[0024] Figure 8 shows a flowchart for calculating the amount of displacement in the optical axis direction of the ROI, taking into account the tilt of the sample. In this case, steps S11 to S15 shown in Figure 8 are performed instead of steps S03 to S07 in the flowchart of Figure 3.

[0025] First, an observation image is acquired in the first position adjustment field of view including the first evaluation region 72 (S11), and the amount of positional displacement in the optical axis direction for the first evaluation region 72 is calculated (S12). This series of processes is the same as the flow shown in Figure 3. Similarly, an observation image is acquired in the second position adjustment field of view including the second evaluation region 73 (S13), and the amount of positional displacement in the optical axis direction for the second evaluation region 73 is calculated (S14). Based on the amount of positional displacement in the optical axis direction for the first evaluation region 72, the amount of positional displacement in the optical axis direction for the second evaluation region 73, and the positional relationship between ROI 71, the first evaluation region 72, and the second evaluation region 73 on the sample surface, the amount of positional displacement in the optical axis direction for ROI 71 is calculated (S15). Here, an example is shown in which the amount of positional displacement in the optical axis direction for ROI 71 is calculated based on two evaluation regions, but the amount of positional displacement in the optical axis direction for two or more evaluation regions may also be used. In addition, the sample cross-section may be assumed to be a plane or a curved surface.

[0026] By setting multiple evaluation areas on the sample 20 in this way and calculating the amount of positional displacement in the optical axis direction of the ROI based on the tilt of the sample 20, it becomes possible to accurately adjust the position (height) in the optical axis direction of the ROI, even when the ROI and the evaluation areas are located far apart.

[0027] Figure 8 shows an example where the ROI has been identified, but the amount of displacement in the optical axis direction may be calculated for multiple evaluation regions in order to search for an ROI. That is, in order to select a flat region as the ROI, the amount of displacement in the optical axis direction is calculated for multiple evaluation regions, and the flatness of the region enclosed by the multiple evaluation regions is evaluated. The flatness may be evaluated by estimating it using a modeled sample shape obtained by fitting the positions of the multiple evaluation regions for which the displacement amounts were calculated to a curved surface, or it may be evaluated from the magnitude of the variability in the displacement amounts of the multiple evaluation regions.

[0028] Figure 9 shows the flow for acquiring correlation data as explained using Figure 5. Figure 10 shows the data (correlation data) acquired by the flow in Figure 9. First, in order to acquire correlation data, the objective lens 22 is demagnetized so that the magnetic history of the objective lens 22 does not affect the magnification (S21). Next, the excitation amount Ex of the objective lens 22 is set to (Ex_S - β) (S22). The excitation amount Ex_S is an excitation amount that is arbitrarily set as the excitation amount that gives the reference height, and in this example, for example, data is acquired in the range of ±β with respect to the excitation amount Ex_S which is set in accordance with the observation magnification. The range in which data is acquired is not limited to this example. In this state, the sample 20 is moved to the focal position of the objective lens 22 with excitation amount (Ex_S - β) by autofocus using the Z movement mechanism of the sample stage 21 (S23), and the Z direction coordinate of the sample stage 21 at that time is stored (S24). Next, the observation image of the evaluation area of ​​the sample 20 is acquired (S25). The evaluation region is the region containing structures with known dimensions, as shown in Figures 4A to 4D. The magnification of the acquired observation image is calculated and stored (S26). Having now obtained data for the case where the excitation amount Ex is (Ex_S - β), the excitation amount is increased by Δε and the data acquisition process is repeated until the excitation amount Ex reaches Ex_S + β (S27, S28).

[0029] At this stage, data 81 to 84 of the correlation data 80 shown in Figure 10 are registered. Data ID 81 is an ID assigned for each measurement, indicating that data was acquired n times while changing the excitation amount of the objective lens 22. Excitation amount 82 is the excitation amount set for the objective lens 22 in the measurement, Z-direction coordinate 83 is the Z-direction coordinate of the sample stage 21 stored in step S24, and magnification 84 is the magnification of the observed image stored in step S26.

[0030] Here, let's assume that when data ID = k, the excitation amount Ex is Ex_S, the Z-axis coordinate is Zk, and the magnification is Mk. If when data ID = i, the Z-axis coordinate is Zi and the magnification is Mi, then the positional displacement amount 85 in the optical axis direction is calculated as (Zi - Zk), and the actual magnification ratio 86 is calculated as (Mi / Mk) (S29). This positional displacement amount 85 in the optical axis direction and the actual magnification ratio 86 correspond to the horizontal axis and vertical axis of the graph shown in Figure 5, respectively. The table of correlation data 80 shown in Figure 10 may be stored directly in the auxiliary storage device 45, or the relationship between the positional displacement amount 85 in the optical axis direction and the actual magnification ratio 86 may be stored in the auxiliary storage device 45 as a mathematical formula. Such correlation data is created for each predetermined set magnification and stored in the auxiliary storage device 45.

[0031] Furthermore, instead of calculating the magnification for each measurement, it is also possible to simply capture an observation image during the measurement and determine the magnification for each observation image after all measurements have been completed. Additionally, while the actual magnification ratio was calculated as correlation data, other indicators such as the difference in magnification (Mi-Mk) could also be used. Any indicator that allows for understanding the correlation between the amount of positional displacement of the sample in the optical axis direction from the reference height and the magnification fluctuation of the observation image is acceptable, and is not limited to the indicators shown on the vertical and horizontal axes of the graph in Figure 5.

[0032] The present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to add, delete, or replace some of the configurations of the embodiments with other configurations. For example, although this embodiment was described using a TEM as an example, the technology of the present invention is also applicable to charged particle beam devices such as STEM (Scanning Transmission Electron Microscope) and SEM (Scanning Electron Microscope) that irradiate a sample with a charged particle beam to form an image. In the case of STEM and SEM, an observed image is formed by scanning a narrowly focused charged particle beam and mapping the signal from a detector that detects signal electrons emitted in response to the interaction between the sample and the charged particle beam according to the scanning position. The image magnification is determined by the scanning range of the deflector equipped in the charged particle optical system, but if the height at which the sample is held is different, the image magnification changes as the scanning range of the charged particle beam on the sample changes, even if the same amount of deflection is applied to the charged particle beam. By applying the present invention, it becomes possible to control the charged particle beam apparatus, such as STEM and SEM, which scan a charged particle beam over a sample, in order to obtain image magnification with high absolute accuracy and high reproducibility.

[0033] 1: Electron source, 2: Extraction electrode, 11: Condenser lens, 12: Condenser diaphragm, 20: Sample, 21: Sample stage, 22: Objective lens, 23: Limiting field diaphragm, 30: Imaging lens, 31: Camera, 40: Control device, 41: Computer, 42: Control system, 43: Processor, 44: Memory, 45: Auxiliary storage device, 46: Output device, 47: Input device, 48: Communication I / F, 49: Bus, 51: Imaging unit, 61: Fourier transform image, 62: Diffraction spot, 63: Pattern, 64: Line and space pattern, 71: Region of interest, 72: First evaluation region, 73: Second evaluation region, 80: Correlation data, 81: Data ID, 82: Excitation amount, 83: Z-direction coordinate, 84: Magnification, 85: Positional displacement in the optical axis direction, 86: Actual magnification ratio.

Claims

1. A charged particle beam apparatus comprising: a sample stage for holding a sample including an evaluation region; a charged particle optical system for irradiating the sample with a charged particle beam; a detector for detecting signal electrons emitted as a result of interaction between the sample and the charged particle beam; and a control system for controlling the sample stage, the charged particle optical system and the detector, wherein the control system calculates the amount of positional displacement of the evaluation region from a reference height based on the magnification of the observation image of the evaluation region generated based on the signal electrons detected by the detector.

2. The charged particle beam apparatus according to claim 1, wherein the charged particle beam apparatus is a transmission electron microscope.

3. The charged particle beam apparatus according to claim 1, wherein the sample stage is equipped with a moving mechanism for moving the sample in the optical axis direction of the charged particle optical system, and the control system has in advance stored correlation data between the amount of displacement of the evaluation region in the optical axis direction from the reference height and the magnification fluctuation of the observed image, calculates the amount of displacement of the evaluation region in the optical axis direction based on the magnification of the observed image and the correlation data, and controls the sample stage based on the calculated amount of displacement.

4. The charged particle beam apparatus according to claim 1, wherein the evaluation region is a region including a structure of known dimensions.

5. The charged particle beam apparatus according to claim 4, wherein the structure of known dimensions is a crystal.

6. The charged particle optical system according to claim 1, wherein the charged particle optical system comprises a deflector for deflecting the charged particle beam, and the magnification of the observation image of the evaluation region is calculated from the change in the observation region when a predetermined amount of deflection is applied to the charged particle beam by the deflector.

7. The charged particle beam apparatus according to claim 1, wherein a plurality of evaluation regions are set on the sample, and the control system estimates the shape of the sample based on the amount of positional displacement in the optical axis direction of the charged particle optical system for each of the plurality of evaluation regions.

8. The charged particle beam apparatus according to claim 3, wherein a region of interest different from the evaluation region is set on the sample, and the control system calculates the amount of positional displacement of the evaluation region in the optical axis direction when acquiring an observation image of the region of interest, and controls the sample stage based on the calculated amount of positional displacement.

9. The charged particle beam apparatus according to claim 8, wherein a plurality of evaluation regions are set on the sample, and the control system calculates the amount of displacement in the optical axis direction of the region of interest based on the amount of displacement in the optical axis direction for each of the plurality of evaluation regions, and controls the sample stage based on the calculated amount of displacement.

10. A method for calculating sample position information for a charged particle beam apparatus, wherein the charged particle beam apparatus comprises a sample stage for holding a sample including an evaluation region, a charged particle optical system for irradiating the sample with a charged particle beam, a detector for detecting signal electrons emitted as a result of interaction between the sample and the charged particle beam, and a control system for controlling the sample stage, the charged particle optical system and the detector, wherein the control system calculates a sample position information amount from a reference height of the evaluation region based on the magnification of the observation image of the evaluation region generated based on the signal electrons detected by the detector.

11. A method for calculating sample position information according to claim 10, wherein the charged particle beam apparatus is a transmission electron microscope.

12. A sample shape estimation method using the sample position information calculation method according to claim 10, wherein a plurality of evaluation regions are set on the sample, and the control system estimates the shape of the sample based on the amount of positional displacement in the optical axis direction of the charged particle optical system for each of the plurality of evaluation regions.

13. A sample position adjustment method using the sample position information calculation method according to claim 10, wherein the sample stage is equipped with a movement mechanism for moving the sample in the optical axis direction of the charged particle optical system, the control system has in advance stored correlation data between the amount of positional displacement of the evaluation region in the optical axis direction from the reference height and the magnification fluctuation of the observed image, calculates the amount of positional displacement of the evaluation region in the optical axis direction based on the magnification of the observed image and the correlation data, and controls the sample stage based on the calculated amount of positional displacement.

14. The sample position adjustment method according to claim 13, wherein a region of interest different from the evaluation region is set on the sample, and the control system calculates the amount of positional displacement of the evaluation region in the optical axis direction when acquiring an observation image of the region of interest, and controls the sample stage based on the calculated amount of positional displacement.

15. The sample position adjustment method according to claim 14, wherein a plurality of evaluation regions are set on the sample, and the control system calculates the amount of positional displacement in the optical axis direction of the region of interest based on the amount of positional displacement in the optical axis direction for each of the plurality of evaluation regions, and controls the sample stage based on the calculated amount of positional displacement.

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